It is well known that the performance of a conventional microphone or, generally, a sound lever meter or acoustical measuring system may be affected by electromagnetic interference (EMI) which may be present in the area of interest as a result of electromagnetic radiation From various sources. Most of existing sound level meters are affected by EMI. More advanced sound meters are equipped with EMI shields. For conventional meters, lacking such protection, it is desirable to assess the effect of EMI on a sound level meter to minimize the resulting systematic measurement error. The more advanced sound level meters (SLMs) are also much more expensive than conventional SLMs.
To assess the presence of EMI and its effect on a sound level meter that has inadequate EMI protection, the level of sound reception must be significantly reduced if not totally eliminated (which is very difficult). An acoustic insulation is needed which would be practically transparent to electromagnetic interference while removing a significant portion of the acoustic signal bandwidth. For practical considerations, a reduction of at least 30 dB would be sufficient.
Various devices are known for protecting a microphone (SLM) from dust or atmospheric conditions. U.S. Pat. No. 3,652,810 is exemplary of such arrangements. They are not designed for sound insulation. On the contrary, the measured sound level should not be significantly reduced by such shields.
It is therefore an object of the invention to provide an acoustically insulative, EMI transparent device suitable for acoustically insulating a sound level meter, typically a conventional microphone having a membrane transducer.
It is another object of the invention to provide a reasonably simple and inexpensive acoustically insulative, EMI transparent device which would provide acoustic insulation of at least 30 dB for most standard SLMs.